Cooling Film Developed at University of Colorado cools 93 watts/meter squared

[jai mitchell]

http://science.sciencemag.org/content/early/2017/02/08/science.aai7899?utm_source=SciPak%20%28updated%202/3/2017%29&utm_campaign=f543794a9b-EMAIL_CAMPAIGN_2017_02_03&utm_medium=email&utm_term=0_10c5e799a3-f543794a9b-126517541

Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling
Yao Zhai et al.

Abstract

Passive radiative cooling draws heat from surfaces and radiates it into space as infrared radiation to which the atmosphere is transparent. However, the energy density mismatch between solar irradiance and the low infrared radiation flux from a near-ambient-temperature surface require materials that strongly emit thermal energy and barely absorb sunlight. We embedded resonant polar dielectric microspheres randomly in a polymeric matrix, resulting in a metamaterial that is fully transparent to the solar spectrum while having an infrared emissivity greater than 0.93 across the atmospheric window. When backed with silver coating, the metamaterial shows a noon-time radiative cooling power of 93 W/m2 under direct sunshine. More critically, we demonstrated high-throughput, economical roll-to-roll manufacturing of the metamaterial, vital for promoting radiative cooling as a viable energy technology.


more here:  https://weather.com/science/environment/news/colorado-boulder-plastic-cooling-study

The research team estimated that 20-square-meters of the film placed on top of an average American house would be able to keep the internal temperature down to 68 degrees on a day when it is nearly 99 degrees outside.

http://www.economist.com/news/science-and-technology/21716599-film-worth-watching-how-keep-cool-without-costing-earth

Preventing something warming up is not, though, the same as cooling it. The key to doing this is the glass beads. Temperature maintenance is not a static process. All objects both absorb and emit heat all the time, and the emissions are generally in the form of infrared radiation. In the case of the beads, the wavelength of this radiation is determined by their diameter. Handily, those with a diameter of about eight microns emit predominantly at wavelengths which pass straight through the infrared “window” in the atmosphere. Since the source of the heat that turns into this infrared is, in part, the building below, the effect is to cool the building.

That cooling effect, 93 watts per square metre in direct sunlight, and more at night, is potent. The team estimates that 20 square metres of their film, placed atop an average American house, would be enough to keep the internal temperature at 20°C on a day when it was 37°C outside.

[jai mitchell]

And we know that atmosphere is only transparent in IR whenot it is bone dry ( poles ) without clouds.

That is not a correct statement, the absorption of IR varies in the spectrum according to the resonance frequencies of the molecular composition.  Water vapor has distinct absorption bands.  these films emit IR at frequencies that are not absorbed by water vapor.  The emission is as though it is operating in space (with 93% of incoming heat energy reflected back out to space).

[silkman]

I hesitate to say it but this appears to be a really "cool" idea. This is from the linked Economist article:

  That cooling effect, 93 watts per square metre in direct sunlight, and more at night, is potent. The team estimates that 20 square metres of their film, placed atop an average American house, would be enough to keep the internal temperature at 20°C on a day when it was 37°C outside.

To regulate the amount of cooling, any practical system involving the film would probably need water pipes to carry heat to it from the building’s interior. Manipulating the flow rate through these pipes as the outside temperature varied would keep the building’s temperature steady. Unlike the cooling system itself, these pumps would need power to operate. But not much of it. Other than that, all the work is done by the huge temperature difference, about 290°C, between the surface of the Earth and that of outer space.   

Can those more expert than me spot a flaw? There has to be one...... Surely....

[DrTskoul]

I hesitate to say it but this appears to be a really "cool" idea. This is from the linked Economist article:

  That cooling effect, 93 watts per square metre in direct sunlight, and more at night, is potent. The team estimates that 20 square metres of their film, placed atop an average American house, would be enough to keep the internal temperature at 20°C on a day when it was 37°C outside.

To regulate the amount of cooling, any practical system involving the film would probably need water pipes to carry heat to it from the building’s interior. Manipulating the flow rate through these pipes as the outside temperature varied would keep the building’s temperature steady. Unlike the cooling system itself, these pumps would need power to operate. But not much of it. Other than that, all the work is done by the huge temperature difference, about 290°C, between the surface of the Earth and that of outer space.   

Can those more expert than me spot a flaw? There has to be one...... Surely....

For any heat transfer from inside the house to the film there has to be a temperature difference. Transfering the heat from the air in the house to the water you would need 5-10 degree differential ( air hotter ) and a very large surface area. Also to transfer the heat from the pipes to the film you need a similar temp differential ( water hotter ) and surface area. Capturing and using such a low quality heat in general requires very large available surface area. At minimum from air inside the house to film , you would need s temperature differential of at least 20 degrees ( film cooler). So if air inside is 20, film has to be at 0? Will check math tomorrow... Too late now..

[TerryM]

DrT
I'd seen an article about this a few days ago that has a few extra tid-bits.


http://www.colorado.edu/today/2017/02/09/newly-engineered-material-can-cool-roofs-structures-zero-energy-consumption


It seems to me as though it will only work when it is directly exposed to the sky. If it was placed under the shingles it would only attempt to heat the shingles. If protected by glass, a nice little greenhouse might result.
Will this material prove resistant to rain sleet and hail, or perhaps the depredation of curious squirrels?


In the article I'd linked to they mention that "even a small amount of directly-absorbed solar energy is enough to negate passive radiation". They therefor mount their product on a reflective surface, similar to double thick aluminum foil. I believe the figures they give in your linked article are the combined result of a lot of reflected solar energy and the much smaller component that is radiated away.


As you've mentioned, to actually lower temperatures within a house you have to have the heat transferring to the heat sink & the thumbnail for this is a delta of 20 degrees F. If it's 120 F outside, the 100 F inside might not be found acceptable.


If it works as advertised it's a very interesting material, but for a residence I think I'd prefer painting my roof white.(and purchasing a good swamp cooler or AC)


Terry

[jai mitchell]

The film takes any heat that it is exposed to and releases it in a specific wavelength that is dependent on the microspheres diameter within the sheet.  There is no top or bottom to this emission so, to direct it out to space a reflective film of silver is coated on the underside of the sheet.  Then the infrared heat radiation at the specific wavelength will radiate out to space with 93% efficiency.

Thin plastic films are very flimsy, it is unclear what long-term solar radiation will do to the material.  Plastics normally left out in the sunshine become brittle due to solar radiation stripping electrons from the material. 

Fortunately, normal high-strength silica based glass transmits infra red completely.  so layering a sheet of this plastic underneath high-strength glass will prevent damage to the material.  This sheet will then cool the surface that it is set onto.  Cooling the surface of a roof by 20C all year around, which would be a problem in the winter I expect.

http://www.edmundoptics.com/contentassets/639fec6d719643a8a3400e1205e6fa84/fig-4-cmi.gif

however, the potential uses of this technology are very great.  for instance, a large basin in clear sky nights could easily cool enough to form ice through radiative cooling.   this ice could then be used to cool a room or building during the heat peak with very little energy spent. 

This material would work on walls as well.

[Neven]

Any potential use for the Arctic? Big rafts full of the stuff to fool the rest of the water into believing it's an ice floe?

[jai mitchell]

none that I can see.   I am afraid that atmospheric geoengineering or other heat processes (like the new one in the guardian today) are the only kinds of impacts that might stave off total sea ice loss in the next few years.  it just won't happen in time but maybe in a decade when we get really scared of climate change.  I see lots of wonderful potential when we actually start to mobilize our vast technological and economic resources to address this existential threat.

[DrTskoul]

And since science most of the time is the ability to extend the findings of others :

Aubrey Jaffer: Radiative Cooling in Hot Humid Climates

The science article material is an optimization of what has been described and patented before. Great science!!
Biggest effect is the reflectance of the sunlight. General blackbody radiation systems implemented previously can provide upwards of 30 - 50 watt/m2 of cooling power for tropical climates. A blackbody at 300K emitts about 450 W/m2 to space (3K).

[TerryM]

Jai


As a very over the hill refrigeration tech I'd been quite dismissive of the film. From your description I can see so many possibilities that it boggles the mind.


Why not flip the panels over in winter & unplug the furnace?
Could a box with the cold sides in be called a refrigerator, even a freezer?
Could a box with the cold sides out be called an oven?
Could these provide energy for both sides of a Sterling engine?
Could lenses direct the IR to a specific location to boil water to spin turbines & generate electricity, even when the sun has set?
Why not power a piezoelectric generator 24/7 with these?

This all sounds far too good to be real


Terry

[jai mitchell]

Jai


As a very over the hill refrigeration tech I'd been quite dismissive of the film. From your description I can see so many possibilities that it boggles the mind.

This all sounds far too good to be real

Terry

Why not flip the panels over in winter & unplug the furnace?
The benefit of the technology is its ability to radiate heat to space, otherwise it serves no warming purpose and is simply a plastic sheet

Could a box with the cold sides in be called a refrigerator, even a freezer?
or a cooler

Could a box with the cold sides out be called an oven?
no it doesn't work that way, it is not hot to the touch, unless it is heated by other sources.  it only radiates heat energy at a specific wavelength that passes easily through the atmosphere to space.

Could these provide energy for both sides of a Sterling engine?
it is not a heat source so, no  As far as providing a heat sink, I suppose it could help significantly, say a watercourse 100 meters long with slow moving water underneath a film cover.

Could lenses direct the IR to a specific location to boil water to spin turbines & generate electricity, even when the sun has set?
It is not creating heat it is converting heat that is already there to a specific wavelength

Why not power a piezoelectric generator 24/7 with these?
Piezoelectric uses mechanical energy (i.e. vibrations) not IR

in other words, consider this technology as a very efficient thermal barrier or even a type of insulation.

[gerontocrat]

CONSEQUENCES!  The last thing the oceans need is more plastic in 'em. Plastic decays into extremely small bits that enter the food web, with potentially disastrous effects. Hence the current campaigns to reduce plastic pollution.
The problem with geo-engineering often seems to be the dreaded Law of Unforseen Consequences.
Sorry to be a wet blanket.0

[andy_t_roo]

Anything comes to thermal equilibrium with its surroundings. You can think of this material as, by being selective about only absorbing or emitting frequencies that the atmosphere is transparent to, being slightly closer to thermal equilibrium with empty space, rather than the sun. The downside : strong emission = strong absorption, so this only works when exposed to the sky, if you point it at anything that naturally emits ir at that frequency (the ground) then it will not cool.

[jai mitchell]

It doesn't have to receive IR radiation to emit IR, the emissions of IR happens as the surface warms, whatever the heating source.

[TerryM]

Why not flip the panels over in winter & unplug the furnace?
The benefit of the technology is its ability to radiate heat to space, otherwise it serves no warming purpose and is simply a plastic sheet
If heat is being radiated into space then that heat must have a source. If your glass plate is not a heat collector or heat sink, then how is it cooling the house? Wouldn't building a shaded home work just as well?


Could a box with the cold sides in be called a refrigerator, even a freezer?
or a cooler
Again, from what you had written, " The film takes any heat that it is exposed to and releases it in a specific wavelength". A portion of the heat that the panel is exposed to must come from inside the box. Since the panel is converting this to IR energy that is being directed away from the interior of the box, the interior most become cooler than the ambient air. This arrangement is commonly referred to as a refrigerator (or freezer).


Could a box with the cold sides out be called an oven?
no it doesn't work that way, it is not hot to the touch, unless it is heated by other sources.  it only radiates heat energy at a specific wavelength that passes easily through the atmosphere to space.
The other source you ask for is the IR emitted by the opposing panel. Anything receiving the IR radiation will be warmed. Hence if two panels face each other while both are spewing out IR radiation, I would expect the facing panels to be heated by the radiation from the other. If anything,(a turkey perhaps), was in the box, it would of course absorb the IR and become heated.


Could these provide energy for both sides of a Sterling engine?
it is not a heat source so, no  As far as providing a heat sink, I suppose it could help significantly, say a watercourse 100 meters long with slow moving water underneath a film cover.
I would prefer one side to be basking in the hot glow of IR emitted by the same panels that are constantly cooling the other side. This film is being marketed as having a cooling effect on whatever they are covering. This has to imply a temperature delta between what is covered and what is not. If this heat energy is converted to a particular range of IR radiation, then the IR radiation will heat any IR opaque surface it comes in contact with.


Could lenses direct the IR to a specific location to boil water to spin turbines & generate electricity, even when the sun has set?
It is not creating heat it is converting heat that is already there to a specific wavelength
It is not creating heat, but it is supposedly removing heat, then converting that heat to IR, which is then radiated away in one direction. Radiating all that energy into the vastness of space sounds noble, but if it could be directed to spin a turbine this might have more down to earth utility.

Why not power a piezoelectric generator 24/7 with these?
Piezoelectric uses mechanical energy (i.e. vibrations) not IR
AAAAAK
Too little sleep, or too much coffee. I thought Peltier & wrote Piezo. My Bad!!

in other words, consider this technology as a very efficient thermal barrier or even a type of insulation.
I fear you are correct and that this probably has less efficacy than a straw bale house. If it actually performed as advertised it would change the world overnight.
20 square meters of R-1000 insulation won't bring a house down to 20 degrees on a 37 degree day, and neither will this. All the insulation in the world won't lower the temperature to 20 degrees if 21 degrees is the nighttime low.

In the link I provided they were proposing it's use as a cooling cover for solar panels. I can't imagine getting much solar output if you cover your panels with a layer of tinfoil, no matter how high tech the foil. It might be however be molded into a very cool chapeau.

Terry
Jai
Terry

[jai mitchell]

it isn't hot to the touch, it does not radiate any more IR radiation than any other object at that same temperature, the benefit is that it radiates the IR at specific distinct wavelengths that penetrate the atmosphere to space and so the surface is cooled by space (effectively). 

without a silver backing the surface is translucent to visible light and so if against a hot solar panel it would warm to the temperature of the solar panel but, instead of radiating infra red along the normal temperature spectrum, it would radiate the heat energy at wavelengths that move to space.  Therefore the surface acts as a conversion layer increasing the efficiency of the normal radiative cooling mechanism found in all warm objects.

again, the films do not emit more heat energy than any other object of the same temperature, it simply emits it in a specific, discrete wavelength that easily penetrates the atmosphere.  (essentially a clear, cold dark night sky).

[TerryM]

it isn't hot to the touch, it does not radiate any more IR radiation than any other object at that same temperature, the benefit is that it radiates the IR at specific distinct wavelengths that penetrate the atmosphere to space and so the surface is cooled by space (effectively). 

without a silver backing the surface is translucent to visible light and so if against a hot solar panel it would warm to the temperature of the solar panel but, instead of radiating infra red along the normal temperature spectrum, it would radiate the heat energy at wavelengths that move to space.  Therefore the surface acts as a conversion layer increasing the efficiency of the normal radiative cooling mechanism found in all warm objects.

again, the films do not emit more heat energy than any other object of the same temperature, it simply emits it in a specific, discrete wavelength that easily penetrates the atmosphere.  (essentially a clear, cold dark night sky).

But it has a silver backing.


The challenge for the CU Boulder researchers, then, was to create a material that could provide a one-two punch: reflect any incoming solar rays back into the atmosphere while still providing a means of escape for infrared radiation. To solve this, the researchers embedded visibly-scattering but infrared-radiant glass microspheres into a polymer film. They then added a thin silver coating underneath in order to achieve maximum spectral reflectance.
[/size][/color]
[/size]http://www.colorado.edu/today/2017/02/09/newly-engineered-material-can-cool-roofs-structures-zero-energy-consumption[/color]

[/size]Terry[/color]

[DrTskoul]

The link I added below ( above ) explains the exact operation of such films vs normal blackbody, which by the way between 0 and 37 C has a peak emission around the atmospheric window 8 - 13 μm - the same as the microbeads

And since science most of the time is the ability to extend the findings of others :

Aubrey Jaffer: Radiative Cooling in Hot Humid Climates

The science article material is an optimization of what has been described and patented before. Great science!!
Biggest effect is the reflectance of the sunlight. General blackbody radiation systems implemented previously can provide upwards of 30 - 50 watt/m2 of cooling power for tropical climates. A blackbody at 300K emitts about 450 W/m2 to space (3K).

[jai mitchell]

It doesn't have to have a silver backing that is an add-on  see the image at the top of the thread.

blackbody radiation at ~37C see red line



The difference is a discrete band of radiation, not a 'peak' as in the normal blackbody spectrum